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NMR study of ferroelastic phase transition of tetramethylammonium tetrabromocobaltate
Journal of Molecular Structure 1119 (2016) 479e483
Contents lists available at ScienceDirect
Journal of Molecular Structure journal homepage: http://www.elsevier.com/locate/molstruc
NMR study of ferroelastic phase transition of tetramethylammonium tetrabromocobaltate Ae Ran Lim a, b, *, Sun Ha Kim c, d a
Department of Science Education, Jeonju University, Jeonju 560-759, South Korea Department of Carbon Fusion Engineering, Jeonju University, Jeonju 560-759, South Korea c Korea Basic Science Institute, Seoul Western Center, Seoul 120-140, South Korea d Department of Chemistry, Kyungpook National University, Daegu 41566, South Korea b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 January 2016 Received in revised form 24 April 2016 Accepted 25 April 2016 Available online 27 April 2016
Static and magic angle spinning (MAS) nuclear magnetic resonance (NMR) experiments were carried out on 1H, 13C, and 14N nuclei in order to understand the structural changes of the N(CH3)4 groups in [N(CH3)4]2CoBr4 near the ferroelastic phase transition temperature TC. The two chemically inequivalent N(CH3)4 groups were distinguished using 13C cross-polarization/(CP)MAS and 14N static NMR. The changes in chemical shifts, line intensities, and the spin-lattice relaxation time near TC can be correlated with the changing structural geometry, which underlies the phase transition. The 14N NMR spectra indicated a crystal symmetry change at TC, which is related to the ferroelastic domain with different orientations of the N(CH3)4 groups. The ferroelastic domain walls were confirmed by optical polarizing microscopy, and the wall orientations were described by the Sapriel theory. The transition to the ferroelastic phase was found to be related to the orientational ordering of the N(CH3)4 groups. © 2016 Elsevier B.V. All rights reserved.
Keywords: [N(CH3)4]2CoBr4 Crystal structure Nuclear magnetic resonance Phase transition Ferroelastics
1. Introduction A large number of crystals of the [N(CH3)4]2MCl4 (M ¼ Zn, Co, Cu, Mn, and Cd) type undergo various successive structural phase transitions with decreasing temperature, including the incommensurate-commensurate phase transition [1e4]. The bromide compounds [N(CH3)4]2MBr4 undergo the novel ferroelastic phase transition from the orthorhombic to monoclinic symmetry below room temperature, with anomalous behavior in the monoclinic [5e9]. Among these, bis(tetramethylammonium) tetrabromocobaltate (II) [N(CH3)4]2CoBr4, undergoes a second-order phase transition at the transition temperature TC ¼ 287.6 K, from an orthorhombic structure with the space group Pmcn to a monoclinic one with the space group P121/c1 [5,6,10]. In the hightemperature phase, the two inequivalent N(CH3)4 cations and the CoBr4 anion are disordered symmetrically with respect to the mirror planes. The CoBr4 tetrahedron is only slightly distorted, while the distortions in the N(CH3)4 tetrahedra are large. The [N(CH3)4]2CoBr4 single crystal at room temperature (phase I) has an orthorhombic structure with lattice constants a ¼ 12.683 Å,
* Corresponding author. E-mail addresses: [email protected], [email protected] (A.R. Lim). http://dx.doi.org/10.1016/j.molstruc.2016.04.089 0022-2860/© 2016 Elsevier B.V. All rights reserved.
b ¼ 9.247 Å, c ¼ 16.052 Å, and Z ¼ 4 [11]. The crystal structure of phase I is shown in Fig. 1 [11]. [N(CH3)4]2CoBr4 has been studied by several experimental methods near the phase transition temperature [5e11]. Its structure and phase transition have been determined by X-ray diffraction [6,11] and dielectric measurements [10], respectively. Recently, static nuclear magnetic resonance (NMR) and magic angle spinning (MAS) NMR measurements of the chemical shifts and spin-lattice relaxations in the rotating frame were carried out for [N(CH3)4]2ZnBr4 [12], which is similar to [N(CH3)4]2CoBr4 in crystal structure. Two chemically inequivalent sites, N(1)(CH3)4 and N(2)(CH3)4, were determined using 13C cross-polarization (CP)/ MAS NMR, and the behaviors of the two N(CH3)4 groups were discussed [12]. However, the characteristic for the inequivalent N(CH3)4 ions in phase I, and the ferroelastic property in phase II, have not been investigated by NMR methods in [N(CH3)4]2CoBr4. Here we studied the temperature dependency of the chemical shifts and intensities in 1H MAS and 13C CP/MAS NMR spectra of [N(CH3)4]2CoBr4, in order to understand its structural change and the mechanism of the phase transition, with a focus on the role of N(CH3)4 groups. The 1H spin-lattice relaxation time T1r in the rotating frame was obtained to understand the molecular motion near TC. The roles of the two chemically inequivalent N(CH3)4
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Fig. 2. DSC thermogram of the [N(CH3)4]2CoBr4. Fig. 1. Crystal structure of [N(CH3)4]2CoBr4 at room temperature. Two inequivalent þ tetramethylammonium ions, N(1)(CH3)þ 4 and N(2)(CH3)4 , and one of the equivalent CoBr2 4 ions are shown. The hydrogen atoms on CH3 are not shown.
groups in [N(CH3)4]2CoBr4 in each phase are discussed based on these results. Ferroelasticity was observed in the 14N static NMR spectra measured in the laboratory frame below TC. The existence of ferroelasticity was confirmed by optical polarizing microscopy, and the domain wall orientation modeled with the Sapriel theory.
were measured using a Bruker DSX 400 FT NMR spectrometer. The static magnetic field was 9.4 T and the Larmor frequency was set to uo/2p ¼ 28.9 MHz. The magnetic field was applied along the crystallographic c-axis. The 14N NMR experiments were performed using a one pulse sequence, and the chemical shifts were obtained relative to the reference signal of NH4NO3. 3. Results and discussion
2. Experimental procedure Single crystals of [N(CH3)4]2CoBr4 were grown by slow evaporation from an aqueous solution which contained CoBr2 and N(CH3)4Br in stoichiometric molar ratio. The [N(CH3)4]2CoBr4 crystals are blue and hygroscopic. The structure of the crystal at room temperature was determined with an X-ray diffraction system at the Korea Basic Science Institute, Seoul Western Center. The single crystal was mounted on a Bruker SMART CCD diffractometer equipped with a graphitemonochromated Mo Ka (l ¼ 0.71073 Å) radiation source. Data collection and integration were performed with SMART (Bruker, 2000) and SAINT-Plus (Bruker, 2001) [13]. The phase transition temperature of the crystal was determined by differential scanning calorimetry (DSC) with a DuPont 2010 DSC instrument at a heating rate of 10 C/min. The MAS NMR experiments were performed using a Bruker DSX 400 FT NMR spectrometer at the Korea Basic Science Institute, Seoul Western Center. The magnetic field was 9.4 T, and 1H MAS NMR and 13C CP/MAS NMR experiments were performed at the Larmor frequencies of 400.13 MHz and 100.619 MHz, respectively. The powder samples were placed in a 4-mm CP/MAS probe. The MAS rate was set to 10 kHz for 1H MAS and 13C CP/MAS, to minimize spinning sideband overlap. The chemical shifts were relative to the reference signal of tetramethylsilane (TMS). The T1r values for 1H were measured using p/2t acquisition, by varying the duration of the spin-locking pulses. The p/2 pulse width used for T1r of 1H was 5 ms, according to the spin-locking field at 50 kHz. The temperaturedependent NMR measurements were carried out in the range from 180 to 400 K. The temperature was controlled to an accuracy of ±0.5 K by the nitrogen gas flow and heater current. The heating rate was 1 C/min, and maintained at a constant value for about 5 min before each NMR measurement. The 14N static NMR spectra in the [N(CH3)4]2CoBr4 single crystal
The [N(CH3)4]2CoBr4 crystal exhibits orthorhombic symmetry with cell parameters a ¼ 12.677 Å, b ¼ 9.236 Å, and c ¼ 16.052 Å, according to our X-ray diffraction results. These values are consistent with those reported by Nishihata and Sawada [11]. The DSC curve reveals an endothermic and an exothermic peak at 282 K during heating and cooling, as shown in Fig. 2. These peaks correspond to the ferroelastic phase transition at a temperature consistent with previous reports [5,6,10]. The NMR spectrum for 1H in [N(CH3)4]2CoBr4 was recorded by MAS NMR at a frequency of 400.13 MHz. At room temperature, the spectrum consists of two peaks at chemical shifts d ¼ 1.19 ppm and
Fig. 3. Chemical shifts of 1H MAS NMR spectra in [N(CH3)4]2CoBr4 as functions of temperature.
A.R. Lim, S.H. Kim / Journal of Molecular Structure 1119 (2016) 479e483
Fig. 4. 1H spin-lattice relaxation times in the rotating frame, T1r, in [N(CH3)4]2CoBr4 as functions of temperature.
12.33 ppm, which are assigned to the methyl protons at two chemically inequivalent sites. The 1H chemical shift changed continuously with temperature across TC (Fig. 3) due to the
Fig. 5. (a) In-situ 13C CP/MAS NMR spectra of [N(CH3)4]2CoBr4 at different temperatures. (b) Chemical shifts of 13C CP/MAS NMR spectra of [N(CH3)4]2CoBr4 as functions of temperature.
Fig. 6. The relative intensities in functions of temperature.
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13
C CP/MAS NMR spectra of [N(CH3)4]2CoBr4 as
changing chemical environment around 1H. The spin-lattice relaxation time T1r in the rotating frame was obtained for the two inequivalent 1H. The nuclear magnetization decay of 1H follows a single exponential function. Thus, T1r can be determined by fitting the traces with the equation M(t)/ M0 ¼ exp(t/T1r), where M(t) is the magnetization with the spinlocking pulse duration t, and M0 is the total nuclear magnetization of 1H at thermal equilibrium [14]. The values of T1r in the rotating frame between 180 and 400 K are shown in Fig. 4. Near TC, T1r exhibits abrupt changes, as phase transition is considered to be due to the change in the structural geometry. At low temperature, the T1r values for the two inequivalent methyl protons in the N(CH3)4 groups show similar trends with temperature, whereas their patterns at high temperature correspond to molecular motions represented by BloembergenePoundePurcell (BPP) theory [14]. At 400 K the two T1r values are nearly the same. Structural analysis of the carbon atoms in [N(CH3)4]2CoBr4 was performed using 13C CP/MAS NMR. The in-situ 13C spectra are shown in Fig. 5(a) at various temperatures, with the spinning sidebands marked with asterisks. The spectra consist of two
Fig. 7. Resonance frequencies of 14N NMR spectra of [N(CH3)4]2CoBr4 single crystals as functions of temperature.
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Fig. 8. Domain wall patterns in (a) phase I (300 K) and (b) phase II (270 K) of [N(CH3)4]2CoBr4 single crystals, obtained using optical polarizing microscopy.
resonance lines for N(1)(CH3)4 and N(2)(CH3)4 at all temperatures. The two inequivalent groups N(1)(CH3)4 and N(2)(CH3)4 are distinguished from the 13C chemical shifts (Fig. 5(b)). At room temperature, the two signals have chemical shifts of d ¼ 243.65 ppm and 93.81 ppm with respect to TMS, and are attributed to the carbons in the inequivalent N(1)(CH3)4 and N(2)(CH3)4 respectively. Across the transition point of 286 K the chemical shift slowly and monotonously decreases with increasing temperature, indicating that the structural geometry of 13C in N(CH3)4 groups changes continuously. The relative intensities of the two signals are shown as functions of temperature in Fig. 6. The intensity of N(2)(CH3)4 is higher than that of N(1)(CH3)4. These results are consistent with the observation that the deformation of N(2)(CH3)4 is greater than that of N(1)(CH3)4, as reported by the Xray diffraction study [15]. Above TC and near 300 K, both intensities decrease abruptly, indicating an order-disorder phase transition. The 14N (I ¼ 1) static NMR spectra were obtained in the laboratory frame at a Larmor frequency of uo/2p ¼ 28.9 MHz. The resonance frequencies of the spectra in a [N(CH3)4]2CoBr4 single crystal are shown in Fig. 7 as functions of temperature. Here, the N(1) and N(2) in N(1)(CH3)4 and N(2)(CH3)4 groups above TC can be distinguished by assuming the quadrupole coupling constant e2qQ/ h and the asymmetry parameter h values in [N(CH3)4]2ZnCl4 [16], which is similar in crystal structure. The patterns of the resonance frequencies changed abruptly at the phase transition temperature, indicating a change in the quadrupole coupling constant of the 14N nuclei. The spectra above TC exhibit two groups of two signals for each of N(1)(CH3)4 and N(2)(CH3)4. Below TC each of the four signals is split into several lines (some lines are missing in Fig. 7, as the low-intensity signals were difficult to detect). This change corresponds to a phase transition with the low-temperature phase having monoclinic symmetry, which is lower in order compared with orthorhombic symmetry. The splitting is indicative of a ferroelastic twin domain with different orientations. In order to confirm the ferroelastic property, the domain structures of the [N(CH3)4]2CoBr4 crystal were observed directly under an optical polarizing microscope. The domain patterns of phases I and II in the crystal are shown in Fig. 8(a) and 8(b) respectively. The domain pattern at 300 K (Fig. 8(a)) does not appear to correspond to the paraelastic of phase I. Below TC the microscopic domain walls of parallel lines are indicative of the ferroelasticity in phase II (Fig. 8(b)). Hence the ferroelastic property of [N(CH3)4]2CoBr4 crystals below 286 K was confirmed by both 14N NMR spectra and optical polarizing microscopy. These results could be described with the point group reported by Sapriel's theory [17] and Aizu [18]. Using the spontaneous strain
tensors given by Aizu and the formulas proposed by Sapriel, we can evaluate the domain wall orientations. In the transition from orthorhombic structure with point symmetry group mmm to monoclinic with point symmetry group 2/m, the domain wall orientations are expressed by the following equations: x ¼ 0, z ¼ 0. These equations of the twin boundaries indicate the mmmF2/m ferroelastic species. During the phase transition, the point group symmetry in the crystal changes from mmm (phase I) to 2/m (phase II). 4. Conclusion The structural change during the ferroelastic phase transition of [N(CH3)4]2CoBr4 was studied using 1H and 13C MAS NMR and 14N static NMR, in terms of chemical shifts, relative intensities, and the spin-lattice relaxation time T1r. The two chemically inequivalent N(1)(CH3)4 and N(2)(CH3)4 groups were distinguished using the 13C and 14N NMR spectra. The changes in the 1H spin-lattice relaxation time T1r and the 13C line intensity near TC are consistent with a structural phase transition. The changes in the structural geometry near TC are different for N(1)(CH3)4 and N(2)(CH3)4; the deformation of N(2)(CH3)4 was greater than that of N(1)(CH3)4. The ferroelastic nature of the phase transition in [N(CH3)4]2CoBr4 was confirmed using 14N NMR and optical polarizing microscopy. In addition, the mmmF2/m species by Sapriel theory was applied to model the domain wall orientations. In summary, this study has revealed the characteristics of two chemically inequivalent N(CH3)4 groups in phases I and II of [N(CH3)4]2CoBr4, and the existence of a ferroelastic domain structure of the N(CH3)4 group in phase II. The main indications to the phase transition were related to the orientational ordering of the N(CH3)4 groups, and the ferroelastic twin domain with different orientations. Acknowledgment This research was supported by the Basic Science Research program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education Science and Technology (2015R1A1A3A04001077). References [1] [2] [3] [4]
H.Z. Cummins, Phys. Rep. 185 (1990) 211. A. Abu El-Fadl, A. El-Korashy, H. El-Zahel, J. Phys. Chem. Solids 63 (2002) 375. F. Hlel, A.B. Rhaeim, K. Guidara, Russ. J. Inorg. Chem. 53 (2008) 785. S.A. Sveleba, I.V. Karpa, I.M. Katerynchuk, O.V. Semotyuk, R.M. Shymkiv,
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[5] [6] [7] [8] [9] [10] [11] [12]
I.M. Kunyo, O.I. Fitsych, J. Appl. Spectrosc. 78 (2011) 228. T. Asaha, K. Hasebe, J. Phys. Soc. Jpn. 63 (1994) 2827. K. Hasebe, H. Mashiyama, S. Tanisaki, K. Gesi, J. Phys. Soc. Jpn. 53 (1984) 1866. M. Iwata, S. Harada, Y. Ishibashi, J. Phys. Soc. Jpn. 65 (1996) 1290. K. Tanaka, T. Shimada, Y. Nishihata, A. Sawada, J. Phys. Soc. Jpn. 64 (1995) 146. K. Gesi, K. Ozawa, J. Phys. Soc. Jpn. 52 (1983) 2440. K. Gesi, J. Phys. Soc. Jpn. 51 (1982) 203. Y. Nishihata, A. Sawada, Acta Cryst. C49 (1993) 1939. A.R. Lim, Solid State Sci. 52 (2016) 37.
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[13] SMART and SAINT-plus v6.22, Bruker AXS Inc., Madison, Wisconsin, USA, 2000. [14] N. Bloembergen, E.M. Purcell, R.V. Pound, Phys. Rev. 73 (1948) 679. [15] K. Hasebe, H. Mashiyama, N. Koshiji, S. Tanisaki, J. Phys. Soc. Jpn. 56 (1987) 3543. [16] J. Dolinsek, R. Blinc, Zeit. Naturforsch 41a (1986) 265. [17] J. Sapriel, Phys. Rev. B 12 (1975) 5128. [18] K. Aizu, J. Phys. Soc. Jpn. 28 (1970) 706.
Contents lists available at ScienceDirect
Journal of Molecular Structure journal homepage: http://www.elsevier.com/locate/molstruc
NMR study of ferroelastic phase transition of tetramethylammonium tetrabromocobaltate Ae Ran Lim a, b, *, Sun Ha Kim c, d a
Department of Science Education, Jeonju University, Jeonju 560-759, South Korea Department of Carbon Fusion Engineering, Jeonju University, Jeonju 560-759, South Korea c Korea Basic Science Institute, Seoul Western Center, Seoul 120-140, South Korea d Department of Chemistry, Kyungpook National University, Daegu 41566, South Korea b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 January 2016 Received in revised form 24 April 2016 Accepted 25 April 2016 Available online 27 April 2016
Static and magic angle spinning (MAS) nuclear magnetic resonance (NMR) experiments were carried out on 1H, 13C, and 14N nuclei in order to understand the structural changes of the N(CH3)4 groups in [N(CH3)4]2CoBr4 near the ferroelastic phase transition temperature TC. The two chemically inequivalent N(CH3)4 groups were distinguished using 13C cross-polarization/(CP)MAS and 14N static NMR. The changes in chemical shifts, line intensities, and the spin-lattice relaxation time near TC can be correlated with the changing structural geometry, which underlies the phase transition. The 14N NMR spectra indicated a crystal symmetry change at TC, which is related to the ferroelastic domain with different orientations of the N(CH3)4 groups. The ferroelastic domain walls were confirmed by optical polarizing microscopy, and the wall orientations were described by the Sapriel theory. The transition to the ferroelastic phase was found to be related to the orientational ordering of the N(CH3)4 groups. © 2016 Elsevier B.V. All rights reserved.
Keywords: [N(CH3)4]2CoBr4 Crystal structure Nuclear magnetic resonance Phase transition Ferroelastics
1. Introduction A large number of crystals of the [N(CH3)4]2MCl4 (M ¼ Zn, Co, Cu, Mn, and Cd) type undergo various successive structural phase transitions with decreasing temperature, including the incommensurate-commensurate phase transition [1e4]. The bromide compounds [N(CH3)4]2MBr4 undergo the novel ferroelastic phase transition from the orthorhombic to monoclinic symmetry below room temperature, with anomalous behavior in the monoclinic [5e9]. Among these, bis(tetramethylammonium) tetrabromocobaltate (II) [N(CH3)4]2CoBr4, undergoes a second-order phase transition at the transition temperature TC ¼ 287.6 K, from an orthorhombic structure with the space group Pmcn to a monoclinic one with the space group P121/c1 [5,6,10]. In the hightemperature phase, the two inequivalent N(CH3)4 cations and the CoBr4 anion are disordered symmetrically with respect to the mirror planes. The CoBr4 tetrahedron is only slightly distorted, while the distortions in the N(CH3)4 tetrahedra are large. The [N(CH3)4]2CoBr4 single crystal at room temperature (phase I) has an orthorhombic structure with lattice constants a ¼ 12.683 Å,
* Corresponding author. E-mail addresses: [email protected], [email protected] (A.R. Lim). http://dx.doi.org/10.1016/j.molstruc.2016.04.089 0022-2860/© 2016 Elsevier B.V. All rights reserved.
b ¼ 9.247 Å, c ¼ 16.052 Å, and Z ¼ 4 [11]. The crystal structure of phase I is shown in Fig. 1 [11]. [N(CH3)4]2CoBr4 has been studied by several experimental methods near the phase transition temperature [5e11]. Its structure and phase transition have been determined by X-ray diffraction [6,11] and dielectric measurements [10], respectively. Recently, static nuclear magnetic resonance (NMR) and magic angle spinning (MAS) NMR measurements of the chemical shifts and spin-lattice relaxations in the rotating frame were carried out for [N(CH3)4]2ZnBr4 [12], which is similar to [N(CH3)4]2CoBr4 in crystal structure. Two chemically inequivalent sites, N(1)(CH3)4 and N(2)(CH3)4, were determined using 13C cross-polarization (CP)/ MAS NMR, and the behaviors of the two N(CH3)4 groups were discussed [12]. However, the characteristic for the inequivalent N(CH3)4 ions in phase I, and the ferroelastic property in phase II, have not been investigated by NMR methods in [N(CH3)4]2CoBr4. Here we studied the temperature dependency of the chemical shifts and intensities in 1H MAS and 13C CP/MAS NMR spectra of [N(CH3)4]2CoBr4, in order to understand its structural change and the mechanism of the phase transition, with a focus on the role of N(CH3)4 groups. The 1H spin-lattice relaxation time T1r in the rotating frame was obtained to understand the molecular motion near TC. The roles of the two chemically inequivalent N(CH3)4
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A.R. Lim, S.H. Kim / Journal of Molecular Structure 1119 (2016) 479e483
Fig. 2. DSC thermogram of the [N(CH3)4]2CoBr4. Fig. 1. Crystal structure of [N(CH3)4]2CoBr4 at room temperature. Two inequivalent þ tetramethylammonium ions, N(1)(CH3)þ 4 and N(2)(CH3)4 , and one of the equivalent CoBr2 4 ions are shown. The hydrogen atoms on CH3 are not shown.
groups in [N(CH3)4]2CoBr4 in each phase are discussed based on these results. Ferroelasticity was observed in the 14N static NMR spectra measured in the laboratory frame below TC. The existence of ferroelasticity was confirmed by optical polarizing microscopy, and the domain wall orientation modeled with the Sapriel theory.
were measured using a Bruker DSX 400 FT NMR spectrometer. The static magnetic field was 9.4 T and the Larmor frequency was set to uo/2p ¼ 28.9 MHz. The magnetic field was applied along the crystallographic c-axis. The 14N NMR experiments were performed using a one pulse sequence, and the chemical shifts were obtained relative to the reference signal of NH4NO3. 3. Results and discussion
2. Experimental procedure Single crystals of [N(CH3)4]2CoBr4 were grown by slow evaporation from an aqueous solution which contained CoBr2 and N(CH3)4Br in stoichiometric molar ratio. The [N(CH3)4]2CoBr4 crystals are blue and hygroscopic. The structure of the crystal at room temperature was determined with an X-ray diffraction system at the Korea Basic Science Institute, Seoul Western Center. The single crystal was mounted on a Bruker SMART CCD diffractometer equipped with a graphitemonochromated Mo Ka (l ¼ 0.71073 Å) radiation source. Data collection and integration were performed with SMART (Bruker, 2000) and SAINT-Plus (Bruker, 2001) [13]. The phase transition temperature of the crystal was determined by differential scanning calorimetry (DSC) with a DuPont 2010 DSC instrument at a heating rate of 10 C/min. The MAS NMR experiments were performed using a Bruker DSX 400 FT NMR spectrometer at the Korea Basic Science Institute, Seoul Western Center. The magnetic field was 9.4 T, and 1H MAS NMR and 13C CP/MAS NMR experiments were performed at the Larmor frequencies of 400.13 MHz and 100.619 MHz, respectively. The powder samples were placed in a 4-mm CP/MAS probe. The MAS rate was set to 10 kHz for 1H MAS and 13C CP/MAS, to minimize spinning sideband overlap. The chemical shifts were relative to the reference signal of tetramethylsilane (TMS). The T1r values for 1H were measured using p/2t acquisition, by varying the duration of the spin-locking pulses. The p/2 pulse width used for T1r of 1H was 5 ms, according to the spin-locking field at 50 kHz. The temperaturedependent NMR measurements were carried out in the range from 180 to 400 K. The temperature was controlled to an accuracy of ±0.5 K by the nitrogen gas flow and heater current. The heating rate was 1 C/min, and maintained at a constant value for about 5 min before each NMR measurement. The 14N static NMR spectra in the [N(CH3)4]2CoBr4 single crystal
The [N(CH3)4]2CoBr4 crystal exhibits orthorhombic symmetry with cell parameters a ¼ 12.677 Å, b ¼ 9.236 Å, and c ¼ 16.052 Å, according to our X-ray diffraction results. These values are consistent with those reported by Nishihata and Sawada [11]. The DSC curve reveals an endothermic and an exothermic peak at 282 K during heating and cooling, as shown in Fig. 2. These peaks correspond to the ferroelastic phase transition at a temperature consistent with previous reports [5,6,10]. The NMR spectrum for 1H in [N(CH3)4]2CoBr4 was recorded by MAS NMR at a frequency of 400.13 MHz. At room temperature, the spectrum consists of two peaks at chemical shifts d ¼ 1.19 ppm and
Fig. 3. Chemical shifts of 1H MAS NMR spectra in [N(CH3)4]2CoBr4 as functions of temperature.
A.R. Lim, S.H. Kim / Journal of Molecular Structure 1119 (2016) 479e483
Fig. 4. 1H spin-lattice relaxation times in the rotating frame, T1r, in [N(CH3)4]2CoBr4 as functions of temperature.
12.33 ppm, which are assigned to the methyl protons at two chemically inequivalent sites. The 1H chemical shift changed continuously with temperature across TC (Fig. 3) due to the
Fig. 5. (a) In-situ 13C CP/MAS NMR spectra of [N(CH3)4]2CoBr4 at different temperatures. (b) Chemical shifts of 13C CP/MAS NMR spectra of [N(CH3)4]2CoBr4 as functions of temperature.
Fig. 6. The relative intensities in functions of temperature.
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13
C CP/MAS NMR spectra of [N(CH3)4]2CoBr4 as
changing chemical environment around 1H. The spin-lattice relaxation time T1r in the rotating frame was obtained for the two inequivalent 1H. The nuclear magnetization decay of 1H follows a single exponential function. Thus, T1r can be determined by fitting the traces with the equation M(t)/ M0 ¼ exp(t/T1r), where M(t) is the magnetization with the spinlocking pulse duration t, and M0 is the total nuclear magnetization of 1H at thermal equilibrium [14]. The values of T1r in the rotating frame between 180 and 400 K are shown in Fig. 4. Near TC, T1r exhibits abrupt changes, as phase transition is considered to be due to the change in the structural geometry. At low temperature, the T1r values for the two inequivalent methyl protons in the N(CH3)4 groups show similar trends with temperature, whereas their patterns at high temperature correspond to molecular motions represented by BloembergenePoundePurcell (BPP) theory [14]. At 400 K the two T1r values are nearly the same. Structural analysis of the carbon atoms in [N(CH3)4]2CoBr4 was performed using 13C CP/MAS NMR. The in-situ 13C spectra are shown in Fig. 5(a) at various temperatures, with the spinning sidebands marked with asterisks. The spectra consist of two
Fig. 7. Resonance frequencies of 14N NMR spectra of [N(CH3)4]2CoBr4 single crystals as functions of temperature.
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Fig. 8. Domain wall patterns in (a) phase I (300 K) and (b) phase II (270 K) of [N(CH3)4]2CoBr4 single crystals, obtained using optical polarizing microscopy.
resonance lines for N(1)(CH3)4 and N(2)(CH3)4 at all temperatures. The two inequivalent groups N(1)(CH3)4 and N(2)(CH3)4 are distinguished from the 13C chemical shifts (Fig. 5(b)). At room temperature, the two signals have chemical shifts of d ¼ 243.65 ppm and 93.81 ppm with respect to TMS, and are attributed to the carbons in the inequivalent N(1)(CH3)4 and N(2)(CH3)4 respectively. Across the transition point of 286 K the chemical shift slowly and monotonously decreases with increasing temperature, indicating that the structural geometry of 13C in N(CH3)4 groups changes continuously. The relative intensities of the two signals are shown as functions of temperature in Fig. 6. The intensity of N(2)(CH3)4 is higher than that of N(1)(CH3)4. These results are consistent with the observation that the deformation of N(2)(CH3)4 is greater than that of N(1)(CH3)4, as reported by the Xray diffraction study [15]. Above TC and near 300 K, both intensities decrease abruptly, indicating an order-disorder phase transition. The 14N (I ¼ 1) static NMR spectra were obtained in the laboratory frame at a Larmor frequency of uo/2p ¼ 28.9 MHz. The resonance frequencies of the spectra in a [N(CH3)4]2CoBr4 single crystal are shown in Fig. 7 as functions of temperature. Here, the N(1) and N(2) in N(1)(CH3)4 and N(2)(CH3)4 groups above TC can be distinguished by assuming the quadrupole coupling constant e2qQ/ h and the asymmetry parameter h values in [N(CH3)4]2ZnCl4 [16], which is similar in crystal structure. The patterns of the resonance frequencies changed abruptly at the phase transition temperature, indicating a change in the quadrupole coupling constant of the 14N nuclei. The spectra above TC exhibit two groups of two signals for each of N(1)(CH3)4 and N(2)(CH3)4. Below TC each of the four signals is split into several lines (some lines are missing in Fig. 7, as the low-intensity signals were difficult to detect). This change corresponds to a phase transition with the low-temperature phase having monoclinic symmetry, which is lower in order compared with orthorhombic symmetry. The splitting is indicative of a ferroelastic twin domain with different orientations. In order to confirm the ferroelastic property, the domain structures of the [N(CH3)4]2CoBr4 crystal were observed directly under an optical polarizing microscope. The domain patterns of phases I and II in the crystal are shown in Fig. 8(a) and 8(b) respectively. The domain pattern at 300 K (Fig. 8(a)) does not appear to correspond to the paraelastic of phase I. Below TC the microscopic domain walls of parallel lines are indicative of the ferroelasticity in phase II (Fig. 8(b)). Hence the ferroelastic property of [N(CH3)4]2CoBr4 crystals below 286 K was confirmed by both 14N NMR spectra and optical polarizing microscopy. These results could be described with the point group reported by Sapriel's theory [17] and Aizu [18]. Using the spontaneous strain
tensors given by Aizu and the formulas proposed by Sapriel, we can evaluate the domain wall orientations. In the transition from orthorhombic structure with point symmetry group mmm to monoclinic with point symmetry group 2/m, the domain wall orientations are expressed by the following equations: x ¼ 0, z ¼ 0. These equations of the twin boundaries indicate the mmmF2/m ferroelastic species. During the phase transition, the point group symmetry in the crystal changes from mmm (phase I) to 2/m (phase II). 4. Conclusion The structural change during the ferroelastic phase transition of [N(CH3)4]2CoBr4 was studied using 1H and 13C MAS NMR and 14N static NMR, in terms of chemical shifts, relative intensities, and the spin-lattice relaxation time T1r. The two chemically inequivalent N(1)(CH3)4 and N(2)(CH3)4 groups were distinguished using the 13C and 14N NMR spectra. The changes in the 1H spin-lattice relaxation time T1r and the 13C line intensity near TC are consistent with a structural phase transition. The changes in the structural geometry near TC are different for N(1)(CH3)4 and N(2)(CH3)4; the deformation of N(2)(CH3)4 was greater than that of N(1)(CH3)4. The ferroelastic nature of the phase transition in [N(CH3)4]2CoBr4 was confirmed using 14N NMR and optical polarizing microscopy. In addition, the mmmF2/m species by Sapriel theory was applied to model the domain wall orientations. In summary, this study has revealed the characteristics of two chemically inequivalent N(CH3)4 groups in phases I and II of [N(CH3)4]2CoBr4, and the existence of a ferroelastic domain structure of the N(CH3)4 group in phase II. The main indications to the phase transition were related to the orientational ordering of the N(CH3)4 groups, and the ferroelastic twin domain with different orientations. Acknowledgment This research was supported by the Basic Science Research program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education Science and Technology (2015R1A1A3A04001077). References [1] [2] [3] [4]
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